Optical transmitter suppressing excess emission of laser diode

ABSTRACT

A laser diode (LD) driver to suppress the excess emission of an LD is disclosed. The LD driver has the shunt configuration with a driving transistor connected in parallel to the LD to shunt the bias current provided to the LD. The driver further provides a protection circuit to divide the bias current when the bias current is active but the driving transistor is turned off at an instant of the power on and off of the LD driver.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present application relates to an optical transmitter, inparticular, the application relates to a circuit for driving asemiconductor laser diode (hereafter denoted as LD).

2. Related Background Art

One type of drivers for an LD has been known as, what is called, ashunt-driver. Various prior arts have disclosed the shunt-drivercircuits. The shunt-driver absorbs a primary portion of the bias currentsupplied to the LD responding to a driving signal provided to an inputof the driver. Thus, the bias current flowing in an LD connected to theoutput of the shunt-driver in a primary portion thereof is shunted tothe driver as leaving a rest portion in the LD. Thus, the LD ismodulated by the driving signal. In order to adjust average power and anextinction ratio of an optical output of the LD, the driver oftencontrols an input bias level thereof.

However, an optical transmitter including the shunt-driver leaves asubject that, in the start of the operation, the activation of the biascurrent for the LD precedes the powering of the driver circuit. Undersuch a situation, the driver circuit absorbs no bias current; that is awhole bias current is supplied to the LD, which results in an excessemission of the LD. Also, at stopping the optical transmitter, thepower-down of the driver circuit occasionally precedes the cut-off thebias current to the LD. In such a case, the whole the bias currentinstantaneously flows in the LD to cause the excess emission. Thepresent application is to provide a technique to prevent such an excessemission of the LD.

SUMMARY OF THE INVENTION

An aspect of the present application relates to an optical transmitterthat includes an LD, a bias current source, and an LD driver. The biascurrent source is connected in series to the LD between the second powersupply and the ground, and powered by the second power supply. The LDdriver includes first and second transistors each powered by the secondpower supply through the bias current source. The first transistor ismade of n-type transistor, biased in the gate thereof by the first powersupply, and powered by the second power supply through the bias currentsource. The first transistor drives the LD by dividing the firstbypassing current from the bias current. The second transistor is madeof p-type transistor, powered by the second power supply through thebias current source. The second transistor divides the second bypassingcurrent from the bias current when the first power supply is inactivebut the second power supply is active.

When only the second power supply is active, the bias current flowing inthe LD causes a forward voltage Vf in the LD, which equivalently causesa drain bias Vds of the second transistor to generate the secondbypassing current. Then, the bias current provided to the LD isdecreased by the second bypassing current, by which the LD escapes fromthe state of the excess emission.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in further detail with reference to theattached drawings in which:

FIG. 1 is a circuit diagram of an optical transmitter according to anembodiment;

FIG. 2 schematically shows a relation of the optical output Pout to theinput driving signal in the shunt driving configuration;

FIG. 3 is a circuit diagram of another optical transmitter comparable tothe optical transmitter shown in FIG. 1; and

FIG. 4 is a circuit diagram of still another optical transmittermodified from that shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Next, some embodiments will be described as referring to drawings. Inthe explanation of the drawings, numerals or symbols same or similar toeach other will refer to elements same or similar to each other withoutoverlapping explanations.

(First Embodiment)

FIG. 1 shows a circuit diagram of an optical transmitter of the firstembodiment. The optical transmitter 10, which is a type of what iscalled as the shunt-driver, primarily includes an LD 11, a first currentsource 12, a driver 13, a voltage source 14, an inductor 15, and acapacitor 16.

The LD 11, which is a type of semiconductor laser diode, generates anoptical signal responding the driving current I_(LD) provided thereto.An anode of the LD 11 couples with the first current source 12 and anoutput terminal 13 b of the driver 13; while, a cathode thereof isdirectly connected to the ground GND. The LD 11 emits light providedwith the driving current I_(LD) derived from a bias current Ibias fromthe first current source 12, which generates a forward voltage Vf ofabout 1.3 V. The first current source 12 generates, powered by thesecond power supply Vcc2, the bias current Ibias to the LD 11, where thebias current Ibias is generally about 50 to 60 mA. The first currentsource 12 in the output thereof is connected to both of the anode of theLD 11 and the output 13 b of the driver 13.

The driver 13 has a type of, what is called, the shunt-driver to drivethe LD 11. The shunt-driver has a function that, shunting a portion ofthe bias current Ibias depending on an input signal Vin to the driver13, which equivalently modulates the LD 11. The driver 13 provides theinput terminal 13 a, the output terminal 13 b, the ground terminal 13 c,and the power supply terminal 13 d. The input terminal 13 a, which isbiased by the voltage source 14 through the inductor 15, receives thedriving signal Vdrv through the capacitor 16. The output terminals 13 bis, as already described; connected to the first current source 12 andthe anode of the LD 11. The ground terminal 13 c is grounded. The powersupply terminal 13 d receives the first power supply Vcc1.

The voltage source 14, which is powered by the first power supply Vcc1,generates a stabilized bias Vg to the input terminal 13 a of the driver13 through the inductor 15. The stabilized bias Vg is greater than apreset threshold Vth and typically about 0.8 V. The bias Vg gives acenter value of the input signal Vdrv, which determines an average ofthe optical signal output from the LD 11 when the input signal has amark ratio of 50%.

The inductor 15 is connected, in one of terminals thereof, to the outputof the voltage source 14; while, the other terminal is connected to theinput terminal 13 a of the driver 13. The capacitor 16 is provided forcoupling the input signal Vdrv with the driver 13 in AC mode. One ofterminals of the capacitor 16 receives the input signal Vdrv; while, theother is connected to the inductor 15 and the input terminal 13 a.

Details of the driver 13 will be described. The driver 13 includes twotransistors, 31 and 33, two resistors, 32 and 34, and a current source35, namely, the second current source.

One of the transistors, the first transistor 31, is a type of then-MOSFET in the present embodiment. The first transistor 31 in thesource thereof is grounded through the ground terminals 13 c. The gateis connected to the input terminal 13 a to receive the input signal Vin;while, the drain is connected to the output terminal 13 b. The firsttransistor 31, responding the input signal Vin, shunts the firstbypassing current Ibp1 from the bias current Ibias. That is, the firsttransistor 31 absorbs the first bypassing current Ibp1 in the drain tothe source. Accordingly, the first transistor 31 is often called as adriving transistor powered by the second power supply Vcc2 through thefirst current source 12.

The resistor 32 operates as a terminator. Specifically, one terminal ofthe resistor 32 is connected to the gate of the first transistor 31, andthe other terminal thereof is grounded. The resistor 32 preferably hasresistance of 50Ω.

The other transistor 33, namely, the second transistor, operates as aprotection circuit for the LD 11 from the excess emission. The secondtransistor 33 is a type of p-MOSFET and connected in parallel to the LD11. The source of the second transistor 33 is coupled with the outputterminal 13 b, the drain thereof is grounded through the ground terminal13 c, and the gate is statically biased through the second currentsource 35 and the second resistor 34.

The second transistor 33 shunts the second bypassing current Ibp2 fromthe bias current Ibias depending on the forward voltage Vf of the LD 11and the gate bias thereof. Specifically, when the source of the secondtransistor 33 becomes higher than the gate bias thereof by a presetamount; the second bypassing current Ibp2 flows in the second transistor33 from the source to the drain. Moreover, the second transistor 33 hasa gate width enough for flowing in a current to suppress the excessemission; specifically, the second transistor 33 is necessary to absorba current of 20 to 30 mA at most. Accordingly, the second transistor 33typically has the gate width of at least 90 μm.

The second resistor 34 operates as a voltage source. That is, oneterminal of the resistor 34 is coupled with the gate of the secondtransistor 33 and the output of the second current source 35; while, theother terminal is grounded. The second resistor 34 has resistance of 10to 20 kΩ. The second current source 35 provides to the second resistor34 a constant current Igp of, for instance, 100 to 200 μA.

The second transistor 33, the second resistor 34, and the second currentsource 35 constitute a protection circuit for the excess emission of theLD 11. The second current source 35 and the second resistor 34constitute a reference generator to generate a shut-off voltage, whichcorresponds to a voltage drop caused in the second resistor 34 by thecurrent Igp coming from the second current source 35. The shut-offvoltage is typically from 1.1 to 1.8 V.

Next, the operation of the optical transmitter 10 will be described.FIG. 2 indicates a relation between the input signal Vin applied to thegate of the first transistor 31, the output current Iout of the driver13, and the optical output Pout obtained from the LD 11 against theoutput current Iout of the driver 13. Under a normal operation whereboth power supplies, Vcc1 and Vcc2, are activated, the voltage source 14biases the input terminal 13 a by providing the static bias Vg thereto.Then, the first bypassing current Ibp1, whose magnitude Iave istypically 20 to 30 mA, flows into the drain of the first transistor 31.As already described, the first bypassing current Ibp1 is divided fromthe bias current Ibias and streamed into the driver 13 from the outputterminal 13 b. As a result, the LD 11 emits light with an average powerPave supplied with the driving current I_(LD) which is subtracted fromthe bias current Ibias by the first bypassing current Ibp1.

Considering the driving signal Vdrv, the driving signals Vdrv providedthrough the coupling capacitor 16 is superposed on the static gate biasVg. That is, referring to FIG. 2, the gate input Vin for the firsttransistor becomes the gate bias Vg in an average thereof with high andlow levels, V_(H) and V_(L), respectively, where a differenceV_(H)−V_(L) becomes substantially equal to the magnitude of the drivingsignal Vdrv; the first bypassing current Ibp1 swings around the averageIave, which resultantly varies the driving current I_(LD) for the LD 11in a phase opposite to the driving signal Vdrv. That is, when thedriving signal Vdrv is in LOW, the first bypassing current Ibp1 is inLOW, the driving current I_(LD) is in HIGH I_(H), and the output opticalpower Pout becomes in HIGH P_(H). On the other hand, when the drivingsignal Vdrv is in HIGH V_(H), the first bypassing current Ibp1 in HIGH,but the driving current I_(LD) for the LD 11 is in LOW I_(L), theoptical output power Pout becomes in LOW P_(L).

Considering the start-up of the optical transmitter 10, the first powersupply Vcc1 occasionally staggers the operation thereof from that of thesecond power supply Vcc2 at the starting. For instance, the first powersupply Vcc1 sometimes delays by a few milliseconds from the activationof the second power supply Vcc2 because the first power supply drivesmany capacitive loads. In such a case, the bias current Ibias is firstsupplied to the LD 11 from the second power supply Vcc2. At thisinstant, the voltage source 14 supplies no gate bias Vg because thefirst power supply Vcc1 is not activated yet; but the gate of the firsttransistor 31 is grounded through the first resistor 32, which turns offthe first transistor 31. Then, no first bypassing current Ibp1 flows inthe drain of the first transistor 31; accordingly, all bias currentIbias is provided to the LD 11 as the driving current I_(LD).

The driving current I_(LD) provided to the LD 11 generates a forwardvoltage Vf in the LD 11, which causes a bias V_(DS) between the sourceand the drain, and the gate bias V_(GS) between the gate and source ofthe second transistor 33; because the gate thereof is grounded throughthe second resistor 34 even the second current source 35 provides nocurrent Igp, then a substantial gate bias V_(GS) is applied to thesecond transistor 33. Thus, as the driving current I_(LD) increases, thegate bias V_(GS) of the second transistor 33 generates and increases thesecond bypassing current Ibp2, which is divided from the bias currentIbias to decrease the driving current I_(LD). Accordingly, the secondtransistor 33, or the second bypassing current Ibp2, operates as thecurrent feedback for the driving current I_(LD) to prevent the LD 11from being provided with an excess current.

Subsequently, activating the first power supply Vcc1, the voltage source14 provides the gate bias Vg to the first transistor 31 to flow thefirst bypassing current Ibp1 in the drain thereof. Moreover, the secondcurrent source 35 begins to provide the current Igp to the secondresistor 34 as the first power supply Vcc1 is activated, which increasesthe gate level thereof and equivalently decreases the gate bias V_(GS)of the second transistor 33. Finally, the second transistor 33 turns offand the second bypassing current Ibp2 is cut off. The driving currentI_(LD) for LD 11 is simply determined by the bias current Ibias and thefirst bypassing current Ibp1.

The power-off sequence of the optical transmitter 10 will be explained.When the optical transmitter 10 is powered off, the second power supplyVcc2 sometimes causes a time lag of several microseconds to severalmilliseconds with respect to the turning off of the first power supplyVcc1 because, for instance, the first power supply Vcc1 connects manyelements with low impedance as the load. In such a case, the firsttransistor 31 turns off because the gate thereof is grounded through thefirst resistor 32, and no drain current, namely, the first bypassingcurrent Ibp1, flows into the drain of the first transistor 31.

Concurrently with the turning off of the first transistor 31, the secondtransistor 33 in the gate thereof is also grounded through the secondresistor 34 because the second current source 35 provides no currentIgp. However, the LD 11 is still provided with the driving currentI_(LD) because the second power supply Vcc2 delays the turning offthereof from that of the first power supply Vcc1 and the driving currentI_(LD) causes the forward voltage Vf in the LD 11, which generates asubstantial source-drain bias in the second transistor 33. Thus, thesecond transistor 33, which is the p-type MOSFET in the embodiment, isprovided with the source bias V_(DS) and the gate bias V_(GS);accordingly, the second transistor 33 flows a substantial current Ibp2into the source thereof, which shunts the bias current Ibias anddecreases the driving current I_(LD). Thus, the LD 11 is protected froman excess current. Finally, the driving current I_(LD) becomes zero asthe second power supply Vcc2 turns off.

FIG. 3 is a circuit diagram of an optical transmitter 100 comparable tothe present embodiment shown in FIG. 1. The optical transmitter 100 hasthe type of the shunt-driver including the driver 113 instead of thedriver 13. The driver 113 of the comparable example provides noprotection circuit.

When two power supplies, Vcc1 and Vcc2, are both powered, thetransmitter 100 operates substantially in a same manner as that of theaforementioned transmitter 10. On the other hand, when the transmitteris powered on and the first power supply Vcc1 delays the activationthereof from that of the second power supply Vcc2, the second powersupply Vcc2 provides the bias current Ibias to the LD 11 withoutdividing the bypassing current Ibp1 from the bias current Ibias becausethe first power supply Vcc1 is not activated yet and the voltage source14 provides no gate bias Vg to the gate of the first transistor 31,which still turns off the first transistor 31. Thus, the LD 11 issupplied with whole of the bias current Ibias as the driving currentI_(LD), which results in an excess emission of the LD 11.

Similarly, in a sequence where the transmitter 100 is powered off, thefirst power supply Vcc1 sometimes in the inactivation thereof precedesthe second power supply Vcc2. In such a situation, the voltage source 14provides no bias Vg to the gate of the transistor 31, which turns offthe transistor 31 and no drain current flows therein. Then, whole of thebias current Ibias provided from the current source 12 is supplied tothe LD 11 without being divided into the bypassing current Ibp1. Then,the LD 11 falls in the state of the excess emission.

Thus, the LD 11 is set in the state of the excess emission during theinstant when the second power supply Vcc2 in the activation thereofprecedes the activation of the first power supply Vcc1 and anotherinstant when the first power supply Vcc1 turns off precedes theinactivation of the second power supply Vcc2; that is, the LD 11 fallsin the state of the excess emission at the instant when only the secondpower supply Vcc2 is activated.

The optical transmitter 10 of the embodiment operates also supplied withtwo power supplies, Vcc1 and Vcc2, which causes the instant same withthe comparable optical transmitter 100; that is, the instant when onlythe second power supply Vcc2 is activated. However, the driver 13 of theembodiment provides the protection circuit comprised of the secondtransistor 33 that absorbs the second bypassing current Ibp2 in thesource thereof depending on the forward voltage Vf of the LD 11generated by the driving current I_(LD). The second bypassing currentIbp2 operates as the current feedback for the driving current I_(LD).Accordingly, even when only the second power supply Vcc2 is activated,the driving current I_(LD) is restricted by the second bypassing currentIbp2 and the LD 11 is protected from the state of the excess emission.

Moreover, the driver 13, exactly, the protection circuit provides thecurrent source 35 and the second resistor 34 to determine the gate biasfor the second transistor 33. Accordingly, after the first power supplyVcc1 is activated, the current source 35 provides the current Igp to thesecond resistor 34 to increase the gate level of the second transistor33, which means that the gate bias V_(GS) of the second transistor 33decreases, which finally turns off the second transistor 33 under thenormal operation and only the first bypassing current Ibp1 is dividedfrom the bias current Ibias. Thus, the second bypassing current Ibp2does not affect the normal operation of the optical transmitter 10.

(Second Embodiment)

FIG. 4 is a circuit diagram of another optical transmitter 10A havingdistinguishable features of providing a driver 13A modified from theaforementioned driver 13 and an inductor 17 connected in series to theLD 11.

The inductor 17 is put between the first current source 12 and the LD11; while, the modified driver 13A provides a current terminal 13 e.That is, the driver 13A provides the second transistor 33 type of thep-MOSFET whose source is connected to the current terminal 13 e not theoutput terminal 13 b as that of the first embodiment. The outputterminal 13 b of the driver 13 is connected to the anode of the LD 11and a downstream terminal of the inductor 17; while, the currentterminal 13 e is connected to an upstream terminal of the inductor 17.That is, the inductor 17 is put between the current terminal 13 e andthe output terminal 13 b of the driver 13.

The operation of the optical transmitter 10A is the same with that ofthe aforementioned optical transmitter 10. The driver 13A extracts thefirst bypassing current Ibp1 from the bias current Ibias as the outputcurrent Iout under the normal operation of the transmitter 10A. While,the driver 13A extracts the sink current Isink from the bias currentIbias at the instant only the second power supply Vcc2 is activated.Specifically, when the second power supply Vcc2 is activated but thefirst power supply Vcc1 is substantially killed, no first bypassingcurrent Ibp1 is divided from the bias current Ibias and the whole biascurrent Ibias flows into the LD 11, which causes the forward voltage Vfin the LD 11. The forward voltage Vf becomes the source-drain bias ofthe second transistor 33 but the gate thereof is grounded, which causesa substantial gate bias V_(GS) in the second transistor 33 and thesource current, namely, the sink current Isink is divided from the biascurrent Ibias. Thus, the state where the whole bias current Ibias flowsinto the LD 11 is prevented.

When the drain of the first transistor 31 and the source of the secondtransistor 33 are commonly connected to the output terminal 13 b as thearrangement of the first embodiment, stray capacitors Cgs of the secondtransistor 33 between the gate and the source and that Css of the secondtransistor 33 between the source and the substrate possibly degrade highfrequency responses of the first transistor 31 because the firstbypassing current Ibp1 is modulated by the driving signal Vdrv whosefrequency often reaches and sometimes exceeds 10 Gbps. The driver 13A ofthe second embodiment provides the current terminal to absorb the secondbypassing current Ibp2, in other words, the second transistor 33 in thesource thereof is not connected to the output terminal 13 b. Moreover,the modified transmitter 13A provides the inductor 17 between the outputterminal 13 b and the current terminal 13 e. Thus, the modified opticaltransmitter 13A isolates the second transistor 33 from the firsttransistor 31. Thus, the high frequency performance of the firsttransistor 31 is not affected from the existence of the secondtransistor 33.

In the foregoing detailed description, the circuits have been describedwith reference to specific exemplary embodiments thereof. It will,however, be evident that various modifications and changes may be madethereto without departing from the broader spirit and scope of thepresent invention. For instance, the embodiments provide transistors, 31and 33, type of MOSFET. However, the embodiments only require that thefirst transistor is the n-type while, the second transistor is thep-type. Thus, an npn-type bipolar transistor and/or an n-type fieldeffect transistor (n-FET) are applicable as the first transistor, while,a pnp-type bipolar transistor and/or a p-type FET are applicable to thesecond transistor. Also, the embodiments provide the first currentsource 12 to provide the bias current Ibias to the LD 11. However, thefirst current source 12 is replaceable to an inductor. That is, the LD11 is biased by the first power supply Vcc1 through the inductor.Accordingly, the present specification and figures are to be regarded asillustrative rather than restrictive.

What is claimed is:
 1. An optical transmitter, comprising: a laser diode(LD); a bias current source configured to provide a bias current to theLD, the bias current source being powered by a second power supply; andan LD driver including, a first transistor made of n-type transistor,biased by a first power supply, and powered by the second power supply,the first transistor driving the LD by dividing a first bypassingcurrent from the bias current, and a second transistor made of p-typetransistor and powered by the second power supply, the second transistordividing a second bypassing current from the bias current when the firstpower supply is inactive and the second power supply is active.
 2. Theoptical transmitter of claim 1, wherein the first transistor and thesecond transistor are commonly connected in an upstream current terminalthereof to the bias current source, and grounded in a downstream currentterminal.
 3. The optical transmitter of claim 1, wherein the firsttransistor is connected in an upstream current terminal thereof to thebias current source through an inductor, and the second transistor isconnected in an upstream current terminal thereof directly to the biascurrent source.
 4. The optical transmitter of claim 1, wherein the LDdriver further includes a current source and a resistor connected inseries to the current source between the first power supply and aground, the resistor causing a gate bias for the second transistor by acurrent provided from the current source.
 5. The optical transmitter ofclaim 4, wherein the second transistor is turned off when the firstpower supply is activated.
 6. The optical transmitter of claim 1,wherein the first transistor is one of an n-MOSFET, an npn transistorand an n-type filed effect transistor (FET), and the second transistoris one of a p-MOSFET, a pnp transistor, and a p-type FET.
 7. The opticaltransmitter of claim 1, wherein the first transistor is biased in a gatethereof by a voltage source powered by the first power supply.
 8. Anoptical transmitter, comprising: first and second power supplies; alaser diode (LD) connected between the second power supply and theground; a first transistor driven by a driving signal and biased by thefirst power supply, the first transistor being connected in parallel tothe LD to divide a first bypassing current from a bias current providedfrom the second power supply to the LD, the LD being provided with adriving current subtracted by the first bypassing current from the biascurrent when the first and second power supplies are active; and asecond transistor configured to generate a second bypassing current, theLD being driven by the driving current subtracted by the secondbypassing current from the bias current when the second power supply isactive but the first power supply is inactive.
 9. The opticaltransmitter of claim 8, further including a current source and aresistor connected in series to the current source between the firstpower supply and a ground, wherein the second transistor in a gatethereof is grounded through the resistor when the first power supply isinactive and biased by a voltage caused by a current provided from thecurrent source and flowing in the resistor.
 10. The optical transmitterof claim 8, wherein the second transistor has a type opposite to a typeof the first transistor.
 11. The optical transmitter of claim 10,wherein the second transistor is one of a p-type MOSFET, a pnp bipolartransistor, and a p-type field effect transistor (FET), and the firsttransistor is one of an n-type MOSFET, an npn bipolar transistor, and ann-type FET.
 12. The optical transmitter of claim 8, wherein the LD isprovided with the bias current from the second power supply through abias current source.
 13. The optical transmitter of claim 8, wherein theLD is provided is the bias current through an inductor.
 14. The opticaltransmitter of claim 13, wherein the first transistor is powered by thesecond power supply through the inductor and the second transistor ispowered directly by the second power supply.